WHAT started out as "industrial action with a halo" in protest at burgeoning workloads has escalated into the most serious confrontation yet between teachers and Tony Blair's Government.
Teachers in eight more local education authorities will next week join around 1,000 schools in London and Doncaster and refuse to cover for absent colleagues in protest over shortages, raising the prospect of even more children being sent home.
The results of ballots in Kent, Manchester and Reading were being announced as The TES went to press.
They will almost certainly follow teachers in Nottingham, Leicester, Middlesbrough, Portsmouth and Southampton, who this week voted overwhelmingly to work to rule.
It's all highly embarrassing for a government that claims to have made education its top priority and David Blunkett, not surprisingly, this week came out fighting.
In the increasingly bitter war of words between him and the unions, the Education Secretary told Radio 4's Today programme : "If you walk out on a class and send kids home, you damage their life chances."
A source close to Mr Blunkett told The TES that the teacher unions were being dishonest with the public and deluding themselves. The source pointed out that the Government had increased salaries and reduced class sizes.
"The first thing we did in government was to stop a budget from the Tories that would have meant 50,000 fewer teachers," he added.
Nevertheless, the National Union of Teachers and the National Association of Schoolmasters Union of Women Teachers believe their 388,000 members must take action against exploitation.
A survey by The TES and the Secondary Heads Association earier this month revealed that the number of teacher vacancies had doubled since September and suggested there could now be nearly 10,000 permanent, unfilled secondary jobs.
"No one takes pleasure from the situation we find ourselves in," said Eamonn O'Kane, deputy general secretary of the NASUWT.
"The Government cannot continue to pretend there is no shortage of teachers," said Doug McAvoy, NUT general secretary. "Our members are exhausted from the additional workload on top of the shortages, coupled with constant new government initiatives."
The parent-teacher associations are sympathetic - if not wholly supportive - of the action.
"It's very difficult to understand what else the teachers can do," said Margaret Morrissey, from their national confederation.
Parents are concerned by the increasing numbers of supply staff who are now teaching their children. One school in London has employed 22 supply teachers in a term. Another in Wolverhampton told a similar tale.
Mrs Morrissey said: "We are as concerned about children being taught by a stream of supply staff - and not having good-quality teachers - as (we are about) the children not being in school."
The action means the two main classroom unions are now involved in two disputes. The first one over workload, was famously described by Nigel de Gruchy, general secretary of NASUWT, as "industrial action with a halo"and involves members refusing to do unnecessary administrative work.
Both demonstrate the depth of unhappiness in the profession, which is now calling for an independent inquiry into pay and conditions.
"One could say the 'halo' has slipped," Mr O'Kane admitted. "But I am not sure that the education of children is being fundamentally harmed.
"I cannot believe that anyone would reasonably argue that children are getting a good education where overstretched teachers are covering classes in subjects that are not their specialist area."